This invention relates to an apparatus for changing over between digital transmission and analog transmission. More particularly, the invention relates to a digital-transmission/analog-transmission changeover apparatus in a communication system in which a digital transmission is performed between a digital terminal and a terminal accommodated by an ISDN, and an analog transmission is performed between a digital terminal and a terminal accommodated by an analog network via an ISDN and an analog network.
Though ISDNs (digital networks) have become widespread in recent years, analog networks still exist. As a consequence, there are cases in which a digital terminal accommodated by an ISDN (1) performs a digital communication with a terminal accommodated by an ISDN and (2) performs an analog communication, via an ISDN and analog network, with a terminal accommodated by an analog network. FIG. 15 is a diagram showing the configuration of a communication system according to the prior art, and FIG. 16 is block diagram illustrating the construction of a terminal adapter according to the prior art.
The communication system shown in FIG. 15 includes an ISDN 1, an analog network 2, a modem 3, a terminal 4 such as a host or telephone accommodated by the analog network, a terminal 5 such as a host or ISDN telephone accommodated by the ISDN, digital terminals 7, 7' of a personal computer or the like accommodated by the ISDN 1, a modem (modulator/demodulator) 8 for modulating digital data, which enters from the digital terminal, into analog data and demodulating an analog signal, which enters from a terminal adapter 9, into digital data, the terminal adapter (TA) 9, which has a digital interface and an analog interface, and a codec (coder/decoder) 10, which is an inter-network interface device provided between the ISDN and the analog network 2. The codec 10 converts 64 kb/s digital data entering from the ISDN 1 into an analog signal and converts the analog signal entering from the analog network 2 into 64 kb/s digital data (e.g., eight-bit data having a sampling frequency of 8 khz).
As shown in FIG. 16, the terminal adapter 9 has a digital interface (DIF) 9a, an analog interface (AIF) 9b, an ISDN interface (IIF) 9c, a digit detector (DLD) 9d for detecting a party's telephone number or the like, a processor (CPU) 9e for executing call processing control, a rate converter (VCT) 9f and a PCM converter (also referred to as a codec) 9g.
The digital interface 9a provides an interface for connecting a digital terminal, and the analog interface 9b provides an interface for connecting a modem or an analog terminal.
The ISDN interface 9c, which is connected to the ISDN 1 in line with a basic rate interface (so-called 2B+D) or first group rate interface (so-called 23B+D) advised by the CCITT, has a B-channel processor (BCP) 9h and a D-channel processor (DCP) 9i. The basic rate interface has two information channels referred to as "B channels" having a transmission rate of 64 kb/s, and one channel, referred to as a "D" channel, for both control signals and information, having a transmission rate of 16 kb/s, for a total of three multiplexed channels.
The rate converter 9f, which is in accordance with advisory V110/V120, etc., of the CCITT, converts the rate (user data rate) of data entering from the digital terminal to the rate (64 kb/s) conforming to the ISDN, and converts the rate (64 kb/s) of data entering from the ISDN to the user data rate. More specifically, the rate converter 9f converts digital data of various rates entering via the digital interface 9a into digital data of 64 kilobits per second in accordance with a predetermined rate matching procedure and sends the digital data to a B channel of the ISDN 1 via the ISDN interface 9c. Further, therate converter 9f converts 64 kb/s digital data entering via the ISDN interface 9c into digital data, which is capable of being received by a digital terminal, in accordance with the above-mentioned rate matching procedure, and sends the digital data to a digital terminal via the digital interface 9a.
The PCM converter 9g converts an analog signal entering via the analog interface 9b into digital data having a rate (64 kb/s) conforming to the ISDN, and sends this digital data to a B channel of the ISDN 1 via the ISDN interface 9b. Further, the PCM converter 9g converts 64 kb/s digital data entering from the ISDN via the ISDN interface 9c into an analog signal and enters the analog signal in the analog interface 9b.